High Energy Density Lithium-ion Batteries Fast Charging Safety Margin:Analysis,Perception And Regulation

With the promotion of fast charging technology for electric vehicles,battery thermal runaway accidents during charging have occurred frequently,becoming a pain point that needs to be solved urgently in the industry.Realizing the safe and fast charging of power batteries is faced with problems such as complex thermal runaway mechanisms,unobservable internal conditions,and insufficient environmental adaptability,which needs to be studied in depth.In this paper,aiming at solving the problem of "the unclear safety margin of fast charging,the inaccurate measurement of safety margin,and the insufficient safety margin at low temperature",studies on safety characteristics and fast charging control of high specific energy lithium-ion batteries are carried out from three levels: mechanism analysis,state perception and operation control.The charging safety margin of batteries is clarified,observed and expanded,enabling safe and fast charging for the vehicle power battery system.The main research contents and results are as follows:In terms of mechanism analysis,a partial cell test method is developed to decouple the complex thermodynamic systems of batteries for the transient thermal runaway process.This method is applied in the process analysis on thermal runaway after fast charging,determining the reaction system that deteriorates thermal stability of power batteries,revealing a novel thermal runaway triggering mechanism that caused by the violent exothermic reaction of active lithium and electrolyte.This mechanism predicts the corresponding relationship between the thermal runaway characteristics and the amount of lithium plating during fast charging,and determines the safety margin of fast charging is no lithium plating.In terms of state perception,confronting the measurement process of the internal potential signal of batteries,the local liquid phase potential deviation mechanism caused by the sensor’s blocking of lithium ion transmission is deduced,and the electrochemical model of the internal potential distribution of the battery is established.This model is applied to the analysis of the potential measurement results of the built-in reference electrode of copper wire with plated lithium,clarifying the generation and evolution principles of the measurement errors for the reference electrode.These principles guide the error suppression and compensation methods for the anode potential measurement,realizing accurate monitoring of electrode potential with reference electrode.In terms of operation management,facing the motor excitation pulse current technology for low-temperature heating of the battery system,the heating current active excitation method based on the battery pack grouping was invented.To realize this method,a set of feasible multi-module circuit topologies for drive system,battery pack heating current patterns and corresponding control strategies are given.Taking the dualmodule circuit topology as an example,the characteristics of high-efficiency and highpower of the system,lossless and rapid heating of the battery,and low noise of the motor are illustruated quantatively.This method creates an optimal mode for low-temperature heating of on-board battery packs based on an electric drive system,with the advantages in heating rate and heating noise are verified in real-vehicle applications.The above research clarifies safety criterion of battery fast charging with no lithium plating,proposed a accurate potential measurement method for sensing lithium plating,and designed a fast heating technology that regulates the margin of lithium plating.These results ensure that the battery charging safety margin is cognizable,observable,and.controllable,systematically realizing the safe and fast charging of the battery system.